Austenitic stainless steel resistant to stress-corrosion cracking



United States Patent AUSTENITIC STAINLESS STEEL RESISTANT T STRESS-CORROSION CRACKING Alexei W. Loginow and Edson H.- Pli'elps, Monroeville Borough, Pa., assignors to United States Steel Corpora tion, a corporation of Delaware No Drawing. Filed Dec. 3, 1962, Ser. No. 241,520 2 Claims. (Cl. 75-128) This invention relates to an austenitic stainless steel which is resistant to stress-corrosion cracking. The steel of our invention is of the type consisting essentially of iron, carbon, chromium, nickel, nitrogen, and the usual steel-making residual elements.

Austenitic stainless steels have had widespread use as constructional materials in nuclear reactors and in the chemical industry, but service experience and laboratory tests have shown that under certain adverse conditions, such as chloride-type environments, stress-corrosion cracking of these steels can occur. Because of the hazards associated with failure of critical components in nuclear reactors and because of the expense involved in replacing components that have failed in the chemical industry, an austenitic stainless steel that is resistant to stress-corrosion cracking is needed. Although stress-relieving treatments, proper design, and process control can be used to minimize the problem of stress corrosion, these preventive measures are usually expensive and are not always reliable and practical.

Accordingly, it is a'principal object of this invention to provide an austenitic stainless steel having a composition inherently resistant to stress-corrosion cracking, including resistance to failure from this phenomenon at elevated temperatures in chloride-type environments.

These and other objects will become apparent from the ensuing discussion of specific examples and tests.

The steel contains sufficient chromium (about 17 percent) for good resistance to corrosion and high-temperature oxidation. Nickel is added to the steel in sufiicient amount to assure an austenitic structure, high ductility, and good cold-working properties. The carbon content is controlled closely at a relatively high level (about 0.08 percent) to provide austenite stabilization and resistance to stress-corrosion cracking. The nitrogen content of the steel is kept at a very low level in the interest of resistance to stress-corrosion cracking. The other elements contained in the new steel, such as manganese, sulfur, phosphorus, silicon, molybdenum, and aluminum are present in the amounts normally present in 18 percent chromium, 8 percent nickel (AISI Type 304) austenitic stainless steels.

The characteristic ranges of proportions of elements making up the steel of the invention are presented in Table I.

Balance iron and residual amounts of other steelmaking elements.

Patented Oct. 4, 1966 to the feriitizing elements,- primarily chromium, to ren der' the steel predominately austenitic.

Table H presents an example of a typical steel of the invention, namely a nominally 18 percent chromium and Spercent nickel base composition, along with representative compositions of similar steels with critical elements falling .outside of the composition range necessary to impart stress-corrosion resistance. The results of comparative tests of these steels are reported in Table 111.

TABLE II.-COMPOSITION OF STEELS WHOSE CORROSION PROPERTIES ARE REPORTED IN TABLE III Steel of Other Steels Invention, percent AISI 304, AISI 304L, percent percent Carbon 0. 07 5 O. 05 0. 029 Manganese-.-" 1. 50 1. 67 1. 28 Phosphorus- 0. 028 0. 024 O. 019 Sulfur- 0. 012 0. 013 0. 015 Silicon 0. 50 0. 49 0. 35 Nickel 8. 0 9. 73 9. 52 Chromium l7. 7 18.8 18. 6 Nitrogen 0. 006 0. 029 0. 040 Molybdenum 0. 22 0. 19

Balance iron and residual amojnts of other steelmaking elements. Not intentionally added-present in residual amounts.

TABLE III.STRESS'CORROSION CRACKING OF AUS TENIC STAINLESS STEELS LN BOILING 42% MgCl:

Steel: Time to failure, hrs.

Steel of invention .Q. 4500 (no failure) A151 304 4 AISI '304L 5 Note. Specimens were annealed at 1950 F., quenched in water, and stressed, throughout test, to of the yield strength.

The above table is illustrative of the superior performance of the austenitic stainless steel ofthis invention as compared to those previously available for comparable applications.

While we have shown and described specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A predominately austenitic stainless steel consisting essentially of:

Percent Carbon 0.075 to 0.1 Manganese 1.0 to 2.0 Phosphorus 0.04 maximum Sulfur 0.03 maximum Silicon 1.0 maximum Nickel 7.0 to 14.0 Chromium 17 to 19 Nitrogen 0.015 maximum with the balance essentially iron, said steel being characterized by inherent resistance to stressacorrosion crackmg.

predominately austenitic. stainless steel consisting References Cited by the Examiner essentlany Percent UNITED STATES PATENTS Carbon 0 075 to 009 2,150,901 3/1939 AIHBSS 7 -123 Manganese 1.4 to 1.6 5 1 Phosphorus 0.02 to 0.03 OTHER REFERENCES S lfu 0 01 to 02 Metal Progress, v01. 80, December 1961, pages 99-102. Silicon 0.4 to 0,6 Transactions of the American Society for Metals, vol. Nickel 8.0 to 12.0 30, page 947. Published in 1942 by theA.S.M., Cleve- Chromium 17.0 to 18.0 I 10 land, Ohio. v Nitrogen 0.010 maximum DAVID RECK, Primary Examiner.

with the balance essentially iron said steel being characterized by inherent resistance to stress-corrosion cracking. WEINSTEIN, Examiner; 

1. A PREDOMINATELY AUSTENTIC STAINLESS STEEL CONSISTING ESSENTIALLY OF: PERCENT CARBON 0.075 TO 0.1 MANGANESE 1.0 TO 2.0 PHOSPHORUS 0.04 MAXIMUM SULFUR 0.03 MAXIMUM SILICON 1.0 MAXIMUM NICKEL 7.0 TO
 14. 0 CHROMIUM 17 TO 19 NITROGEN 0.015 MAXIMUM WITH THE BALANCE ESSENTIALLY IRON, SAID STEEL BEING CHARACTERIZED BY INHERENT RESISTANCE TO STRESS-CORROSION CRACKING. 